Electron Transfer Process and
Membrane Bioenergetics
APh/BE161: Physical Biology of the Cell
Winter 2009
Electron Transfer
Rob Phillips
Plan of Lecture
Announcements: nature of estimates, office hour changes, HW2.
First part: electron transfer – phenomenology, simplest model
Second part: membrane bioenergetics
Schematic of the Electron Transfer
Process
Schematic of the charge transfer
process after optical excitation.
(A) The initial events in a reaction center create a charge
separation. A pigment-protein complex holds a chlorophyll
molecule of the special pair (blue) precisely positioned so that
both a potential low-energy electron donor (orange) and a
potential high-energy electron acceptor (green) are immediately
available. When light energizes an electron in the chlorophyll
molecule (red electron), the excited electron is immediately
passed to the electron acceptor and is thereby partially
stabilized. The positively charged chlorophyll molecule then
quickly attracts the low-energy electron from the electron donor
and returns to its resting state, creating a larger charge
separation that further stabilizes the high-energy electron. These
reactions require less than 10-6 second to complete. (B) In the
final stage of this process, which follows the steps in (A), the
photosynthetic reaction center is restored to its original resting
state by acquiring a new low-energy electron and then
transferring the high-energy electron derived from chlorophyll
to an electron transport chain in the membrane. As will be
discussed subsequently, the ultimate source of low-energy
electrons for photosystem II in the chloroplast is water; as a
result, light produces high-energy electrons in the thylakoid
membrane from low-energy electrons in water.
Abstracting the Electron Transfer Process
Schematic of the charge transfer process
after optical excitation.
First, focus on a single electron transfer reaction.
Next, examine the free energy available by setting up an electrochemical potential
gradient.
The Machines of the Membrane
We already talked about cyanobacteria. Most
familiar photosynthetic organisms are plants.
They have internal organelles devoted to
photosynthetic process (these organelles are
thought to be endosymbionts – how do we know?).
Chloroplast structure is rich and fascinating, and
features a complex membrane system dividing the
chloroplast into three distinct spaces.
Thylakoid membranes are a challenge to our
understanding of biological membrane
morphology.
Measuring the Rate of Electron Transfer:
the Case of Azurin
http://www.chem.ox.ac.uk/icl/faagroup/azurin.jpg
(From our own Prof. Harry Gray – Langen, Gray et al., Science 1995)
Measuring the Rate of Electron Transfer:
the Case of Azurin
Protein engineering permits construction of
donor-acceptor pairs at different distances from
each other.
Measure the rate of electron transfer as a
function of distance. Data is nuanced!
(From our own Prof. Harry Gray – see his papers in PNAS)
“Changes in Redox Potential During
Photosynthesis”
The energetics of the light-induced
reactions have been worked out.
The redox potential for each molecule is indicated by
its position along the vertical axis. Note that
photosystem II passes electrons derived from water to
photosystem I. The net electron flow through the two
photosystems in series is from water to NADP+, and
it produces NADPH as well as ATP. The ATP is
synthesized by an ATP synthase that harnesses the
electrochemical proton gradient produced by the
three sites of H+ activity that are highlighted in
Figure 14-48. This Z scheme for ATP production is
called noncyclic photophosphorylation, to distinguish
it from a cyclic scheme that utilizes only photosystem
I (see the text).
Harnessing the Proton Gradient to Make
ATP
Figure 14-15. ATP synthase. (A) The enzyme is
composed of a head portion, called the F1 ATPase,
and a transmembrane H+ carrier, called F0. Both
F1 and F0 are formed from multiple subunits, as
indicated. A rotating stalk turns with a rotor
formed by a ring of 10 to 14 c subunits in the
membrane (red). The stator (green) is formed from
transmembrane a subunits, tied to other subunits
that create an elongated arm. This arm fixes the
stator to a ring of 3α and 3β subunits that forms
the head. (B) The three-dimensional structure of
the F1 ATPase, determined by x-ray
crystallography. This part of the ATP synthase
derives its name from its ability to carry out the
reverse of the ATP synthesis reaction—namely, the
hydrolysis of ATP to ADP and Pi, when detached
from the transmembrane portion. (B, courtesy of
John Walker, from J.P. Abrahams et al., Nature
370:621–628, 1994. © Macmillan Magazines Ltd.)
Harnessing the Proton Gradient to Make
ATP
http://www.k2.phys.waseda.ac.jp/F1movies/F1Step.htm